Simplify RANDOM_LOADABLE by removing the ability to unload a LOADABLE
random(4) implementation. The most common use for a RANDOM_LOADABLE module
is to supply one's own FIPS random module. That can be done once, at boot.
Swapping modules on the fly doesn't seem especially useful.
This removes the need to hold a lock over the sleepable module calls
read_random and read_random_uio.